1. Field of the Invention
The present invention relates to a heat-sensitive recording material that can be used in a wide spectrum of fields including printers for computer output and calculators, recorders for medical instruments, low-speed and high-speed facsimiles, automatic ticket machines, heat-sensitive photography, handy terminals, and labels for the POS system.
2. Description of the Related Art
Various types of recording materials have been proposed in which a heat-sensitive recording layer that contains as main components a colorless or light-colored leuco dye and a developer that produces color upon contact with the dye is provided on a substrate such as paper, synthetic paper, plastic film or the like, so that developing reactions between the leuco dye and developer effected by application of heat or pressure are utilized. These types of heat-sensitive recording materials require no troublesome treatments like developing and fixing, offering such advantages as shorter recording time with a relatively simple device, low noise level, and low costs. These advantages have enabled them to be available not only for copying of books and documents, but also as recording materials for use in a variety of fields including computers, facsimiles, ticket machines, label printers, recorders, and handy terminals.
What is demanded for heat-sensitive recording materials is quick, highly dense developing, with high robustness in the developed image and background.
In regard to attempts to achieve increased sensitivity, method have been proposed (see Japanese Patent Application Laid-Open (JP-A) Nos. 59-5093 and 59-225987) in which the thermal conductivity of the substrate is 0.04 kcal/m-hr-° C. and various types of hollow microparticles (resin, glass, aluminosilicates or the like) are used as the intermediate layer as disclosed in JP-A No. 55-164192. However, in these cases, it is difficult to form a uniform intermediate layer and the surface readily becomes uneven, leading to poor resolution (dot reproducibility) in the formed image. Furthermore, a method has been proposed (see JP-A No. 63-281886) in which styrene acrylic resin and polystyrene resin are used as the partition materials of the above-described hollow microparticles, and an intermediate layer is formed, the main components of which are non-foamed hollow microparticles with a hollow ratio of 30% or more (JP-A No. 02-214688). However, even in this case, adequate insulating effects cannot be obtained because the hollow ratio is low, so that the highly sensitive heat-sensitive recording material current being sought cannot be obtained.
Furthermore, in recent years, such recording materials have come to be abundantly used in fields where fidelity of recorded images is deemed critical, such as labels and receipts. Accordingly, recording materials are in demand that have high resistance against water and acidic substances in foods, and oils and plasticizers in organic polymer materials used in packages.
There have been attempts to overcome the aforementioned drawbacks for instance by providing a protective layer on the heat-sensitive recording layer. In particular, it has been proposed that polyvinyl alcohols or modified polyvinyl alcohols be used as the resin for the protective layer, and that these polyvinyl alcohols and a waterproofing agent be used together as the protective layer.
For example, JP-A No. 08-151412 discloses using a hydrazine compound and a diacetone group-containing polyvinyl alcohol, but when they are used in an overcoat of the heat-sensitive recording material, the waterproof reaction is promoted in their coating solution followed by unwanted increase in viscosity with time. In addition, JP-A No. 11-314457 proposes that a diacetone-modified polyvinyl alcohol be used in the resin of the protective layer and that a hydrazine compound be contained in the heat-sensitive color-developing layer, but the problems arise that the waterproof capabilities of the protective layer are insufficient, the viscosity of the coating solution on the heat-sensitive color-developing layer increases and developing of the heat-sensitive color-developing layer is inhibited by the hydrazide compound. In addition, in JP-A No. 10-87936, a waterproofing method is proposed that uses water-soluble amines, hydrazide compounds and polyvinyl alcohol copolymers containing diacetone acryl amide as a monomer. However, when they are used in an overcoat of the heat-sensitive recording material, amines undesirably affect the heat-sensitive color-developing layer to cause coloring in the background, pH control with amines becomes difficult and, depending on the added amine amount, viscosity increases conversely.
Regarding increase in viscosity, JP-A No. 2002-283717 attempts to solve this problem by using a hydrazide compound as a crosslinking agent for a polyvinyl alcohol having a reactive carbonyl group, and also by incorporating a basic filler.
When heat-sensitive recording materials that use hydrazide compounds and polyvinyl alcohol containing a reactive carbonyl group are used, however, an image printed with aqueous ink for flexography is easily peeled off by external force after long-time exposure to water.
Meanwhile, the curtain coating method has received attention for its advantages including significant reduction in expenditures involved in drying equipment and energy, which are achieved by increased coating speed and simultaneous multilayer coating associated with recent demands for increased productivity. JP-A No. 2003-182229 discloses producing a heat-sensitive recording layer by curtain coating in order to obtain a heat-sensitive recording material with excellent sensitivity, quality and matching properties with a thermal head. However, this patent literature remains silent with respect to a heat-sensitive recording material which offers excellent printing suitability and head matching properties and with which high-speed coating is possible.